1. Field of the Invention
The present invention relates to an antenna system used in traffic communication systems.
2. Discussion of Background Information
The present application claims priority under 35 U.S.C. .sctn.119 of Patent Application No. 1055/95-6, filed in Switzerland on Apr. 12, 1995.
Antennas basically perform the task of transformation quadripoles, which cause the adaptation between the wave impedance of the antenna lead and the wave impedance of the free space and which transform an electrical oscillation transmitted through the antenna lead into an electromagnetic wave. In this case the antenna acts as a resonator. The equivalent circuit of a loss-free antenna operated in resonance only consists of its radiation resistance. According to C. Dorf in "The Electrical Engineering Handbook", CRC Press Inc., Boca Raton, Fla., 1993, chapter 36.1, page 864, this radiation resistance for a dipole, which can be determined by the calculation of the electrical and magnetic fields E, H, is approximately 73 Ohm. It is furthermore described at a different place that the effective length of a dipole can be increased by increasing the end capacity of the dipole (end loading). By means of this it is therefore possible to tune a relatively short dipole to signals of greater wavelength. For example, an antenna A3 is represented in FIG. 1, whose effective length was increased by connecting a capacitor Ca3. The wire connecting the terminals of the capacitor Ca3 with each other therefore acts as an inductivity and is schematically identified as the coil La3. It is known that maximal opposite phase voltages occur at the ends of the dipole (capacitive zone), and maximal current between these ends in the inductive zone or in the coil La3. For transmitting data between two transmitter stations, their antennas can be coupled, for example via the magnetic field. In FIG. 1, the antenna A3 is coupled in this way with an antenna A2 of the same kind. Therefore a current in the coil La3 causes a magnetic field, by means of which a current is induced in the coil La2 of the antenna A2. The distance between the antennas La2 and La3 is preferably selected to be less than the wavelength of the transmitted signals. Losses, which are created if electrical fields must pass through strong damping layers, are prevented in this way.
The antennas A2, A3, the source impedance Rq in the transmitter and the balance resistor Rl in the receiver are appropriately matched for the transmission of signals of a maximal signal strength. Because of this it is possible to transmit signals of a high voltage level to the antenna A2. But in this case the antennas A2 and A3 act as narrow-band resonant circuits which only permit signals from a narrow frequency range to pass. But often a greater bandwidth is required for information channels in which increased amounts of data are to be transmitted. The bandwidths can be increased, for example by damping the resonant circuits. However, this also affects the damping behavior of the transmission channel, so that it is necessary, because of the increase in bandwidth of the transmission channel, to tolerate greater signal damping. This damping is correspondingly higher if the bandwidths are large with respect to the transmission frequency or to the center frequency fm in the resonant circuits. The bandwidth of 1 MHz can probably still be satisfactory at a center frequency of 30 MHz, but with center frequencies below 10 MHz it can hardly be realized without the creation of increased transmission losses. To compensate the occurring losses it is therefore necessary to provide a correspondingly higher transmitting power.
However, with traffic engineering communications systems with ground-restricted and mobile communications units in particular, the permissible transmitter output is limited. It must furthermore be assumed that these systems will have increased bandwidth requirements in the future.